The invention relates generally to the field of source measure units and specifically to a sample-and-hold feedback circuit for eliminating transients. A source measure unit (SMU) is an electronic instrument that is capable of providing a constant voltage ("sourcing") while measuring current and providing a constant current while measuring voltage. Features that are desirable in this type of instrument are high resolution and dynamic range in both sourcing and measuring, fast operation, and the absence of unexpected output transients. In order to simultaneously provide high resolution and large dynamic range, it is necessary to equip the instrument with multiple ranges for both voltage and current. The changing of these ranges during testing, however, can cause unwanted transients on the output of the instrument, especially when done quickly. This effect is particularly acute when the static output level is non-zero.
The most common consequence of these transient glitches is a temporary shift of the operating characteristics of the device-under-test. For example, the SMU applies a voltage to the gate-source junction of a MOSFET transistor while another instrument is measuring the drain-source resistance of the device. If the SMU produced an output transient during a range change operation, the energy of that transient would cause the highly capacitive gate-source junction to shift its voltage. This could substantially change the drain-source resistance value of the transistor even though the DC voltage level on the gate-source junction has not changed. Consequently, data taken immediately after the transient on the resistance versus voltage relationship would be in error. Moreover, if the SMU was configured to have a very low current compliance, that is, current drive capability, the time duration of the error could be substantial, as the gate-source capacitance would be discharged to its previous voltage level at a gradual rate. A less common, but more serious, consequence also appears in MOSFET testing. If the gate-source junction is being operated at a voltage that is close to its maximum rated limit, a transient spike could produce a voltage large enough to permanently damage the sensitive gate oxide material of the device.
FIG. 1 illustrates a simple amplifier system, which is analogous of prior art SMU design topology. Assuming that the setpoint signal from the D/A converter is 1V, and the values of R1 and R2 are equal, that is, R1/R2=1, this configuration will yield an output voltage of 2V. Now consider the situation where the ratio of R1/R2 changes to 9, but the desired output level is the same 2V. In order for this to be true, the setpoint signal must decrease to 0.2V. In fact, it must decrease at exactly the same rate and at exactly the same time that the resistor ratio increases in order for the output to show no movement. In practice, however, these operations are typically executed by a microprocessor in serial fashion. That is, one signal changes, followed by the other. Even if there is very little time between the operations, any perturbation in the difference between the signals will result in significant output movement due to the high gain of the amplifier. In theory it might be possible to design analog circuitry that equates the rate of change of the two elements. In practice, however, this would be very difficult to implement, especially over a large number of ranges.
To circumvent this issue, previous designs have implemented an algorithm in which the setpoint is set to zero prior to the switching of the feedback elements. Once the new feedback in is place, the setpoint is reprogrammed for the proper output. This scheme does succeed in lowering glitching, because the output is less sensitive to changes in the feedback elements when little or no voltage is across them. There are some applications, however, where the unexpected movement of the output to zero during testing is not acceptable. It would be preferable for the output to truly maintain its programmed level during the range change process.